Dispenser-type cathode and method of making



R. LEVl March 17, 1959 DISPENSER-TYPE CATHODE AND METHOD OF MAKING Filed April 29, 1957 -THORlUM-IMPREGNATED REFRACTORY-METAL CATHODE INVENTOR. 4 ROBERTO LEVI a I f,

AGENT United States Patent DISPENSER-TYPE CATHODE AND METHOD OF MAKING Roberto Levi, New York, N. Y., assignor to North American Philips Company, Inc., New York, N. Y., a corporation of Delaware Application April 29, 1957, Serial No. 655,901 11 Claims. (Cl. 313346)' This invention relates to dispenser-type cathodes and methods of making same, and in particular to cathodes especially useful in high power, microwave tubes.

In certain types of high power magnetrons operating as continuous wave oscillators, the cathode is subjected to so-called back bombardment, which involves the impingement thereon of returning primary electrons and positive ions. This back bombardment heats up the cathode and causes it to assume very high temperatures of the order of 1400 to 1800 C., totally independent of the temperature produced by the heater for the cathode itself.

' In many such tubes, oscillation is initiated by heating the cathode to a temperature at which sufficient primary electron emission occurs, at which time the back bombardment of the cathode builds up its temperature to higher values-so that the heater filament for the cathode may be ultimately extinguished, and the cathode stabilizes in the high temperature range indicated above. As a consequence, for a cathode to operate in such an environment, it is essential that it be able to operate properly at these relatively high temperatures. Previous dispenser-type cathodes which include an emissive material based upon a barium composition are unsuitable for such applications because, at the high temperatures involved, the life of the cathode is unduly shortened.

One object of the invention is a new dispenser-type cathode capable of operating at higher temperatures than previous dispenser cathodes employing a barium composition.

Another object of the invention is a new dispenser-type cathode suitable for application in high power, microwave tubes with a severe back bombardment condition.

Briefly, the dispenser-type cathode of the invention comprises an inherently self'supporting, refractory-metal body containing pores with thorium metal filling at least part of said pores. The cathode is made, in accordance with the invention, by impregnating the desired pores of a sintered, refractory-metal body with thorium metal from a melt.

The invention will now be described in greater detail with reference to the accompanying drawing, in which the sole figure shows, in cross-section, a dispenser-type cathode in accordance with the invention suitable for incorporation in a magnetron oscillator.

Referring now to the drawing, there is shown therein a hollow, cylindrical-type cathode suitable for incorporation in a conventional magnetron oscillator. As will be observed, the cathode comprises a solid, strong, rigid, self-supporting, refractory-metal matrix or body 1 which contains pores. 4 In some or all of the pores of the refractory-metal body is thorium metal. Within the hollow body is mounted a heating filament 2. The outer surface 3 of the body is its emitting surface. In operation, the heater raises the temperature of the body to a suitable value, for example, about 1650 C., at which temperature thorium vapor ditiuses through the pores or grain bound- .ariesof the refractoryfmetal body and forms a thin monoatomic layer of thorium on the emitting surface 3 of the 2,878,409 Patented Mar. 17, 9

cathode. This monoatomic thorium layer reduces greatly the work function of the subjacent refractory-metallayer so that copious electrons are emitted therefrom at said; elevated temperature. As the thorium metal layer evaporates during the life of the cathode, it is continuously replenished by further diffusion of the thorium metal from the interior of the refractory-metal body. The endroftlife, occurs. when the entire supply of thorium metal inside the refractory-metal body is exhausted. The significantdilference of the above-described cathode over previous dispenser-type cathodes using as emitting material a barium composition is the fact that the operating temperature of. a thorium-on-tungsten type. of emitter is severalhum dred degrees centigrade above that of barium-on-tung sterr, and thus such cathodes of the invention canbe raised to very high temperatures by back bombardment, for example, without detrimentally affecting its operation.

The manufacture of the above cathode will, now. be described in greater detail. The first step is the preparae tion of, an inherently self-supporting, refractory-metal body which is strong and rigid. As a suitable refractory, metal, tungsten, or alloys of tungsten and molybdenum, for example, in a 5050 ratio, are preferred, though, most other refractory metals, including suchas tantalum, rhen, ium, and hafnium, may also be used either alone or in combination with one or more of, the others. In the following description, the manufacture of a tungsten body will be described. To obtain a strong tungsten body con: taining pores, a technique as described in United States Patent No. 2,669,008 may be employed. in particular, tungsten, powder of, suitable particle size and distribution as described in the patent is compacted to form a body, and the body then sintered at a temperature above the melting point of thorium metal. For the greatest strength and to maintain dimensional stability, a v temperature-of about 2200 to 2600 C. is employed. Firing for twenty minutes at a temperature in the middle of that range produces a body with adensity of about 83 to 84 percent of the density of a solid, non-porous, tungsten member Also, a majority of the pores in said body will be interconnected, though this,-as will be explained later, isjnot absolutely necessary. While the tungsten bar, which will warp during the sintering, may be machined to shape by copper impregnation as described in said patent, this cornplicatedand expensive step may be eliminated in accordance with a further feature of the invention that will be described later. 1 The thus-sintered tungsten bar, which may for example, if not machined to size, have a cross? section of inch by inch and a length of about 1% inches, is then placed in contact with an amount of thorium metal powder, and the bar and thorium powder next heated in an inert atmosphere such as helium tothe melting point of the thorium metal, which is about, 1809'" C. brightness. In about five minutes at this temperature, the bar with the dimensions above described will be fully impregnated with thorium metal. Of course, larger bars will. require longer impregnation times and conversely smaller bars shorter impregnation times. In general, the time of impregnation is not critical. If the bodyhas been previously machined to the shape and size desired, it is then ready for mounting in the electron tube desired.

The impregnation of the strong, refractory-metal body can be accomplished simultaneously With its mounting on a suitablesupport. This may be done by placing the re fractory-metal body on top of a small quantity of thorium metal, in turn placed on top of a refractory-metal support, such as tantalum. The assembly is then heated inaninert atmosphere, such as helium, until the thorium its, which results in the molten thorium partially or fully n.- pregnating the porous body, and the remainingor-excess thorium serving as a bond and thus brazing the porous body to the underlying support.

One of the features of the invention is the discovery that a thorium-metal-impregnated tungsten bar made as described above is readily machined by carbide or highspeed steel tools. As a matter of fact, machinability of the thorium-impregnated tungsten bar is at least as good as that of a copper-impregnated bar of the same density. Thus, it is no longer necessary to machine the refractory-metal, body prior to its impregnation with the electron emissive material. On the contrary, the sintered tungsten bar, warped due to shrinkage by its high temperature firing, may be impregnated directly with thorium metal, and then subjected to a machining operation to produce cathodes of the desired shape and dimensions. So, for example, to make the cathode illustrated in the drawing, the bar may be first turned-down in a lathe to the required outside diameter and then sliced to the desired length. Then, the hollow portion of the body can be formed by drilling or boring a hole through the center. Thus, cathodes in accordance with the invention are readily made in a simple and economical manner.

It is another feature of the invention that the cathode of the invention is very flexible and can be tailored to suit conditions present in many widely different applications. Possible modifications have to do with surface treatments of the resultant body or with control of the density of the refractory-metal body. For instance, there are magnetrons in which the total anode current includes a portion due to primary emission of the cathode anda portion due to secondary emission as a consequence of-back bombardment. For those applications where a high secondary emission coefiicient at the cathode emitting surface is required, it may be obtained by subjectingthe cathode, prior to its assembly in the tube, to an oxidation treatment, which thus produces on the cathode surface both a thorium oxide and a tungsten oxide layer, of which the latter is then removed to prevent sublimation in the tube. In a specific example, which is ,only illustrative, the thorium-metal-imoregnated tungsten body may be fired in air at about 500 C. for about [one minute to produce the oxide layers. Then, the body is placed in a vacuum furnace, and heated at 1000 C. "in vacuum, at which temperature the tungsten oxide at -,thesurface is preferentially sublimated and thus re moved from the cathode. In this way, the secondary 'emission coefiicient of the surface of the cathode of the invention may be improved.

' There are also applications where it is essential that the rate of evaporation of the thorium in the cathode be minimized and this for several reasons, among which are changes in the electrode geometry, increase in sparking or corona, and possible contamination of other nonemitting electrodes in the tube. There are several ways :in Which the cathode of the invention can be modified to cut down its rate of thorium evaporation. For example, this may be done with a cathode made as described above by subjecting the cathode, prior to its assembly in a tube, to a surface treatment for removing the thorium metal in the pores adjacent the emitting surface. For example, the cathode may be immersed in a concentrated hydrochloric acid solution for a time sufficient to leach out the thorium from the surface pores 'of the body. T hus, interposed between the emitting surface and the thorium metal supply is an empty space through which the thorium atoms must first diffuse before reaching and depositing on the emitting surface. This isolation of the supply of thorium metal from the emitting surface will reduce the rate of evaporation.

A further way of reducing the evaporation rate of the thorium is to carbonize the surface of the refractorymetal body, which may be done by heating the body in an atmosphere containing hydrocarbon gasses, e. g., naphthalene, whereby the carbon decomposition product diffuses into the surface of the body to form a thin carbide skin.

The above result may be also achieved in another fashion. This involves employing a particle size and distribution for the tungsten powder, and a sintering tern perature and time, all of which is within the province of those skilled in the metallurgical art, such that a very dense refractory-metal body or matrix is produced. For example, densities of values between 87 and 92% are suitable. With such high densities, a high percentage of the pores in the interior are non-connected. Melting thorium metal into such a matrix leads to the presence of thorium metal in interior pores not connected by other pores to the surface. This odd result occurs because the tungsten metal has a small solubility in molten thorium but not in solid thorium. Thus, some of the pores initially closed off by tungsten metal are opened by the molten thorium literally melting its way in. But, upon solidification of the thorium, the dissolved tungsten is reprecipitated out and in many cases again closes-off the recently opened pores. The resultant construction is one in which interior pores of the body not connected by other pores to the emitting surface contain thorium metal, in addition to, of course, surface-connected pores of the body. The thorium in these surface-connected pores may be leached out with hydrochloric acid. Thus not only is the thorium on the interior of the cathode isolated from the emitting surface but its only connection thereto is via grain boundaries of the refractory-metal body. Since diffusion of thorium through grain boundaries is at a lower rate than through open and connected pores, a further reduction in thorium evaporation with an accompanying increase in life of the cathode results. While the latter technique is best suited to .very dense refractory-metal bodies, it may alsobe applicable with less dense bodies. However, in general, the density of the refractory-metal body should always exceed 70% in value as otherwise excessive thorium evaporation will result. Thus, in all cases, the thorium metal will con stitute no more than about 30% by volume of the com pleted body.

It will also be appreciated from the foregoing that types of cathodes other than the cylindrical type illustrated can also be made in accordance with the invention. So, for example, the planar-type cathodes are also possible.

As may have been indicated before, the cathode above described has the advantages that a very strong, hard, inherently self-supporting member constituted of a single matrix is produced. Thus, no matter what the heating environment to which it is exposed, cracking and other defects in the body will not result. Further, the cathode is readily machined to a wide variety of shapes and sizes. Finally, the manufacture of the cathode is very flexible and it can be tailored to suit the require ments of a particular application.

While the invention has been described in connection with cathodes for microwave tubes, its extreme flexibility and the unusual feature of its machinability make them also suitable in another form for other types of electron tubes, such as the ordinary power or transmitting tube. In particular, not only is the thorium-impregnated, tungsten body machinable, but it can also be swaged and rolled or drawn to form wires from the initial bars. In this way, it is possible to obtain filament wires which comprise a tungsten matrix containing a relatively high percentage of up to 25% of thorium metal in pores of the matrix. This construction has several important advantages over the ordinary thoriated-tungsten filament. First and most important of all, the thorium material in the filament is in the metal form and not the oxide. Thus. reduction of the oxide to the metal necessary in the ordi nary thoriated-tungsten filament becomes unnecessary, thus eliminating high-temperature flashing to activate. The manufacture of such filament wires is thus simplified. The second advantage stems also from the form of the thorium in the refractory-metal matrix, which is metal rather than oxide. Since the presence of the metal does not inhibit drawing, there is no upper limit concerning the amount of thorium that may be included in the tungsten base, and by the incc rporation of larger amounts of thorium, a longer life may ensue. This elimination by the construction of the invention of a previous limitation of the prior art cathodes makes possible a far wider use of the thoriated-tungsten type of emitter than was heretofore possible, and represents an important contribution to the cathode art.

While the invention has been described in connection with specific examples and embodiments, other modifications thereof will be readily apparent to those skilled in the art without departing from the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. A dispenser-type cathode adapted for high temperature operation comprising an inherently-self-supporting, refractory-metal body containing pores, said porous, refractory-metal body having a density exceeding the value of 70% of the density of a solid, non-porous refractorymetal body of the same overall dimensions, and thorium metal in a portion of the pores of said body.

2. A dispenser-type cathode comprising an inherently self-supporting, refractory-metal, porous body, and thorium metal in the pores of said body and constituting not more than 30% by volume of the body.

3. A cathode as set forth in claim 2 wherein the body is of tungsten.

4. A dispenser-type cathode comprising a fully-sintered, strong, hard, tungsten-containing, porous body, and thorium metal in pores of said body unconnected 'by other pores to the surface of the body.

5. A dispenser-type cathode comprising a fully-sintered, strong, hard, tungsten-containing, porous body, and thorium metal in interior pores of said body, but the surface pores of the body being free of thorium.

6. A dispenser-type cathode comprising a fully-sintered, strong, hard, tungsten-containing, porous body, thorium metal in pores of said body, and a thorium oxide layer on the surface of the body.

'7. A dispenser-type cathode comprising an inherently- 6 self-supporting, refractory-metal, porous body, thorium metal in the pores of said body and constituting not more than 30% by volume of the body, a supporting member for said impregnated body, and a layer of thorium bonding the impregnated body to the supporting member.

8. A method of making a dispenser-type cathode comprising fully sintering a compacted tungsten powder member to form a self-supporting, tungsten body containing interconnecting pores, impregnating the pores of said body with thorium metal from a melt, and treating the surface of the impregnated body to remove thorium from surface pores of the body.

9. A method of making a dispenser-type cathode comprising fully siutering a compacted tungsten powder member to form a self-supporting, tungsten body containing interconnecting pores, impregnating the pores of said body with thorium metal from a melt, and oxidizing the impregnated body to form a thorium oxide layer on its surface.

10. A method as claimed in claim 8 wherein the treating step comprises subjecting the surface of the body to the action of an acid solution to leach out thorium from surface pores of the body.

11. A method of making a dispenser-type cathode comprising fully-sintering a compacted, tungsten-powder-containing member to form a strong, dense body containing pores, impregnating pores of said body with thorium metal from a melt, oxidizing the impregnated body to form a surface layer containing tungsten oxide and thorium oxide, and removing the tungsten oxide portion of the layer leaving a surface layer of thorium oxide on the body.

References Cited in the file of this patent UNITED STATES PATENTS 2,669,008 Levi Feb. 16, 1954 2,808,530 Katz Oct. 1, 1957 FOREIGN PATENTS 705,126 Great Britain Mar. 10, 1954 

2. A DISPENSER-TYPE CATHODE COMPRISING AN INHERENTLY SELF-SUPPORTING, REFRACTORY-METAL, POROUS BODY, AND THORIUM METAL IN THE PORES OF SAID BODY AND CONSTITUTING NOT MORE THAN 30% BY VOLUME OF THE BODY. 